The invention relates to a TV-IF circuit comprising a balanced video signal path arranged between an IF-input and a video signal output, and a balanced sound signal path arranged between the IF-input and a sound signal output, these paths including a video mixing stage and a sound mixing stage, respectively.
Such a TV-IF circuit is known as a TV-IF circuit for quasi-parallel sound processing and is described in the publication "Aufbereitung des Fernsehtonsignals mit den integrierten Schaltungen TDA 2545 und TDA 2546 nach dem Quasi-Paralleltonverfahren" pulbished in the series "Valvo Entwicklungsmitteilungen", November 1980.
An IF-TV signal applied to the known TV-IF circuit is separated therein into a video and a sound signal. To that end the TV-IF circuit comprises two filter circuits, connected to the IF-input, one filter circuit suppressing the sound carrier of the TV-IF signal and being included in the video signal path and the other filter circuit suppressing the video information with the exception of the picture carrier and being included in the sound signal path. As a result thereof the video signal path contains virtually no sound signal components which may disturb the video signal after demodulation in the video mixing stage and the sound signal path contains virtually no video signal components which may disturb the sound signal after intercarrier mixing of the sound carrier and the picture carrier in the sound mixing stage. During this intercarrier mixing, the frequency of the sound signal modulated on the sound carrier is converted into a sound intermediate frequency which corresponds to the picture-sound carrier spacing in the TV-IF signal. The IF-sound signal thus obtained is not affected by unwanted frequency shifts of the tuning oscillator.
Because of the separated signal processing, the video and sound output signals of the known TV-IF cirucit are disturbed to a lesser extent than those of a TV-IF circuit in which signal processing is effected which is used in common for video and sound signals. In such a so-called TV-IF circuits for intercarrier sound processing, which are also described in the above-mentioned publication, the whole TV-IF signal, optionally after partial suppression of the sound carrier, is applied to the video mixing stage. Thereafter mixing of the TV-IF signal with the picture carrier is effected in the video mixing stage, as a result of which the demodulation of the video signal and, simultaneously, a conversion of the sound signal frequency into the said sound intermediate frequency is obtained in response to an intercarrier mixing of the sound carrier and the picture carrier. In contrast with TV-IF circuits for quasi-parallel sound processing, the demodulated video signal and the IF sound signal are both available here at the same output of the video detector for further signal processing and, because of the common mixing in the video mixing stage, each of the two signals comprises residual components of the other signal.
TV-IF circuits for quasi-parallel sound processing are mainly used in TV-receivers which must satisfy high quality requirements as regards picture display and sound reproduction. Partly due to the high quality requirements which also the other receiver circuits must satisfy, such high-quality TV-receivers are generally comparatively expensive and are only sold in a limited number. Consequently, the production of TV-IF circuits for quasi-parallel sound processing is limited. As a result thereof and also because of the fact that the circuit itself if rather complicated, such circuits are in the present state of the art much more expensive than TV-IF circuits for intercarrier sound processing.
In contrast therewith, the TV-IF circuits for intercarrier sound processing are less complicated and particularly suitable for use in TV-receivers on which lower quality requirements are imposed. The lower quality requirements make it possible to use for the entire TV-receiver simple and cheap receiver circuits so that TV-receivers of this type can be much cheaper than the first-mentioned high-quality TV-receivers. The demand for the cheap TV-receivers is comparatively high and consequently also the production of the last-mentioned TV-IF circuits.